RFR: 8311864: Add ArraysSupport.hashCode(int[] a, fromIndex, length, initialValue)

[Pavel Rappo]

On Thu, 13 Jul 2023 08:44:02 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

>>> For an arbitrary int[], unconditional vectorization might be unwarranted or puzzling.
>> 
>> Could you clarify this statement? Thanks a lot.
>
>> > For an arbitrary int[], unconditional vectorization might be unwarranted or puzzling.
>> 
>> Could you clarify this statement? Thanks a lot.
> 
> My mental model is that Arrays.hashCode is to ArraysSupport.vectorizedHashCode as Arrays.mismatch to ArraysSupport.vectorizedMismatch. In either pair, the first method is applicable to some generic array, whereas the second method is applicable to an array that would definitely benefit from vectorized processing.
> 
> When I see a call to ArraysSupport.vectorizedHashCode, it reads like a statement: this particular computation must be vectorized. Now, was it author's intention, or there simply were no alternatives to compute hash code of an array subrange or with a different initial value?

> @pavelrappo Actually I look at the implementation of `ArraySupport.vectorizedHashCode` and it does branching and only uses vector instructions if the length is sufficiently large. Please do benchmarking if you are uncertain with performance characteristics of functions. Thanks.

Are you sure you are talking about `vectorizedHashCode` and not `vectorizedMismatch`? While the latter indeed branches on array length, the former consists of simple for-loops, which might not be the actual thing anyway because the whole method is `@IntrinsicCandidate`.

Regardless of any performance characteristics, the call to something named `vectorizedHashCode` might needlessly draw attention to something that does not deserve it.
 
My mental model might be wrong, but let me give you a stream analogy. Would you not pause and think if you unexpectedly saw this?

    ...
    .stream()
    ...
    .parallel()
    ...

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PR Comment: https://git.openjdk.org/jdk/pull/14831#issuecomment-1633888417